The use of an elongated liquid filled sensor tube which operates a diaphragm to in turn operate a water valve to extinguish a fire is shown in Wacker, U.S. Pat. No. 868,307 patented Oct. 15, 1907. Here a coiled thermostat is filled with a "confined expandable liquid such as mineral oil . . . " which when heated will actuate a diaphragm. Another type of sensor tube is, for example, illustrated in Lindberg, U.S. Pat. No. 3,277,860, which has encapsulated in it a pressurized gas which is released when heated. This is termed a capillary sensor tube.
In both of the above sensor devices, there may be problems with false alarms or limitations in reliable operation under severe ambient conditions. For example,for use in an engine compartment of an automobile, the system must work at a temperature range from -40 to +300.degree. F.; at the :same time, because of the relatively high temperatures in an engine compartment, it cannot trigger accidentally even under relatively high temperatures.